The manpage says:

The sums are computed as described in FIPS-180-2. When checking, the input should be a former
output of this program. The default mode is to print a line with checksum, a space, a character
indicating input mode ('*' for binary, ' ' for text or where binary is insignificant), and name
for each FILE.

By adding the filename to the generated sha256 file, a user would be
able to download an iso, the sha256 file, then use sha256 -c [file].iso.sha256 to verify the file.

This fixes nixos-homepage#224

Additional notes

Breaking assumptions

This could break assumptions where the .sha256 files were previously used as they were. I have briefly looked and couldn't find evidence of such use.

Testing

I couldn't test the entire changes in-place, but here's how I tested them:

Using this test.pl file,

~/.../nixos/nixos-channel-scripts $ nix-shell -p perl -p perlPackages.FileSlurp
[nix-shell:~/.../nixos/nixos-channel-scripts]$ ./test.pl
[nix-shell:~/.../nixos/nixos-channel-scripts]$ cd tmp/
[nix-shell:~/.../nixos-channel-scripts/tmp]$ ls
testing  testing.sha256
[nix-shell:~/.../nixos-channel-scripts/tmp]$ sha256sum -c testing.sha256
testing: OK

So at least, the basic code is expected to work.

Am I right in assuming there is no test suite for the mirroring script?